To develop methods for in vivo imaging of CNS receptor and signal transduction mechanisms, radiolabeled ligands with appropriate SAR requirements are being produced and used to develop in vivo animal models, and will lead ultimately to studies in human subjects. Synthetic organic chemistry techniques are used to produce cold-labeled derivatives of candidate compounds to determine SAR in displacement assays using standard beta (C-14 or H-3) or gamma (I-125) emitting radioligands. If cold-labeled compounds retain activity, techniques are developed for rapid radiosynthesis. Positron (F-18, C-11, I-124) emitting derivatives are used in initial in vivo studies to determine blood brain barrier penetration and in vivo activity; both autoradiographic and external imaging (high resolution small animal PET) techniques are used. Tracer kinetic (bolus or equilibrium) models have been developed in rodents and primates, and validated using activation and displacement techniques. Synthesis and biodistribution of a F-18 labelled Forskolin analogue, designed for visualization of adenylate cyclase activity in vivo has been reported (Journal of Fluorine Chemistry, 101(2), 297-304, 2000). PET studies using C-11 Raclopride, a D2 dopamine receptor antagonist, and amphetamine in stuttering subjects is scheduled to begin in October, 2001. This method makes it possible to evaluate both pre- and postsynaptic dopaminergic mechanisms within the CNS. Clinical responses to dopamine (DA) antagonists from patients with developmental stuttering have suggested that the pathophysiology of this disorder may involve an abnormality of central DA systems. Pilot studies evaluating voltage sensitive compounds, which permit direct visualization of transmembrane potentials in the CNS, for use in PET have been completed, and studies using these compounds in non-human primates are planned. Studies of CNS uptake and processing of thyroid hormone, using I-124 labeled T3, have been initiated as well.